"EMS/Surge Protector": I don't want crappy electricity getting anywhere near my trailer so built in protection is not for me. A built in unit as shown will not detect a problem upstream from the device; i.e. a short between the external shore power plug and the detector. A short in the wiring coming from the shore power plug could cause a fire and the shown device will not detect that.
I purchased a device that checks for over/under current, voltage, etc. and is also a surge protector. First thing I do upon arriving at a camp site is plug it into the electrical pedestal and let it do it's thing; if everything is OK then and only then will I connect the trailer to the pedestal. I leave the device plugged into the pedestal and the trailer shore power is plugged into the device. In so doing if there is a surge, over or under voltage, sometimes called a brownout, it will be detected, shut off the power if necessary until the problem is resolved, and everything in the trailer will be protected.
My advice is to protect the electrical system in the trailer before external shore power is connected. The device I use is a Southwire Black 34930 Surge Guard 30A-Full Protection Portable with LCD Display. It also comes in a50A version. It's available on Amazon.